Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower). Sodium lights are used, e.g., to light highways—driver response time, however, significantly decreases with lower CRI Ra values (for any given brightness, legibility decreases with lower CRI Ra). See Commission Internationale de l'Eclairage. Method of Measuring and Specifying Colour Rendering Properties of Light Sources, CIE 13.3 (1995) for further information on CRI.
Aspects related to the present inventive subject matter can be represented on either the 1931 CIE (Commission International de I′Eclairage) Chromaticity Diagram or the 1976 CIE Chromaticity Diagram, both of which are well-known and readily available to those of ordinary skill in the art.
The CIE Chromaticity Diagrams map out the human color perception in terms of two CIE parameters x and y (in the case of the 1931 diagram) or u′ and v′ (in the case of the 1976 diagram). The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors. The 1976 CIE Chromaticity Diagram is similar to the 1931 Diagram, except that the 1976 Diagram has been modified such that similar distances on the Diagram represent similar perceived differences in color.
Since similar distances on the 1976 Diagram represent similar perceived differences in color, deviation from a point on the 1976 Diagram can be expressed in terms of the coordinates, u′ and v′, e.g., distance from the point=(Δu′2+Δv′2)1/2, and the hues defined by a locus of points which are each a common distance from a specified hue consist of hues which would each be perceived as differing from the specified hue to a common extent.
The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=A λ−5/(e(B/T)−1), where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody and A and B are constants. Color coordinates that lie on or near the blackbody locus yield pleasing white light to a human observer. The 1976 CIE Diagram includes temperature listings along the blackbody locus. These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally blueish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants which produce light which is on or near the blackbody locus can thus be described in terms of their color temperature.
Many methods are known for allowing a lighting device to be adjustable in color temperature, including using a variable combination of warm white and cool white light sources, using red, green and blue light sources. However, all these methods generally provide low to medium CRI Ra.
Light emitting diode lamps have been demonstrated to be able to produce white light with component efficacy >150 L/W and are anticipated to be the predominant lighting devices within the next decade. See e.g., Narukawa, Narita, Sakamoto, Deguchi, Yamada, Mukai: “Ultra-High Efficiency White Light Emitting Diodes” Jpn. J. Appl. Phys. 32 (1993) L9 Vol. 45, No. 41, 2006, pp. L1084-L10-86; and on the World Wide Web nichia.com/about_nichia/2006/2006—122001.html.
Many systems are based primarily on LEDs which combine blue emitters+YAG:Ce or BOSE phosphors or Red, Green and Blue InGaN/AlInGaP LEDs; or UV LED excited RGB phosphors. These methods have good efficacy but only medium CRI or very good CRI and low efficacy. The efficacy and CRI tradeoff in LEDs is also an issue in the lighting industry with regard to fluorescent illumination. See Zukauskas A., Shur M. S., Cacka R. “Introduction to Solid-State Lighting” 2002, ISBN 0-471-215574-0, section 6.1.1 page 118.
While some luminaires use yellow sodium light, the majority of illumination uses light that is colored more naturally, similar to the color or color temperature of natural sources, including the color emitted by burning fuels, incandescent sources or daylight.
The term “white light” or “whiteness” does not clearly cover this range of colors as it is plain that a candle flame and other incandescent sources are yellowish, i.e., not completely white. Therefore the color of illumination is generally and better defined in terms of correlated color temperature (CCT) and needs to also be defined as to its proximity to the planckian black body locus (BBL) in addition to its CCT.
CRI Ra is the most commonly used metric for measuring color quality today. This CIE standard method (see, e.g., Commission Internationale de l'Eclairage. Method of Measuring and Specifying Colour Rendering Properties of Light Sources, CIE 13.3 (1995)) compares the rendered colors of 8 reference color swatches illuminated by the test illumination to the rendered color of the same swatches illuminated by reference light. Illumination with a CRI Ra of less than 50 is very poor and only used in applications where there is no alternative for economic issues. Lights with a CRI Ra between 70 and 80 have application for general illumination where the colors of objects are not important. For some general interior illumination, a CRI Ra>80 is acceptable.
A light with color coordinates within 4 MacAdam step ellipses of the planckian locus and a CRI Ra>85 is more suitable for general illumination purposes. CRI Ra>90 is preferable and provides greater color quality.
Some of the most commonly used LEDs in solid state lighting are phosphor excited LEDs. In many instances, a yellow phosphor (typically YAG:Ce or BOSE) is coated on a blue InGaN LED die. The resultant mix of yellow phosphor emitted light and some leaking blue light combines to produce a white light. This method typically produces light >5000K CCT and typically has a CRI Ra of between ˜70 and 80. For warm white colors, an orange phosphor or a mix of red and yellow phosphor can be used.
Light made from combinations of standard “pure colors,” red, green and blue, exhibit poor efficacy due primarily to the poor quantum efficiency of green LEDs. R+G+B lights also suffer from lower CRI Ra, in part due to the narrow full width at half maximum (FWHM) values of the green and red LEDs. Pure color LEDs (i.e., saturated LEDs) usually have a FWHM value in the range of from about 15 nm to about 30 nm.
UV based LEDs combined with red, green and blue phosphors offer quite good CRI Ra, similar to fluorescent lighting. Due to increased Stokes losses, however, they also have lower efficacies.
The highest efficiency LEDs today are blue LEDs made from InGaN. Commercially available devices have external quantum efficiency (EQE) as great as 60%. The highest efficiency phosphors suitable for LEDs today are YAG:Ce and BOSE phosphor with a peak emission around 555 nm. YAG:Ce has a quantum efficiency of >90% and is an extremely robust and well tested phosphor. Using this approach, almost any color along the tie line between blue and yellow is possible as shown in FIG. 2.
If the portion of the lumens of blue light is approximately greater than 3% and approximately less than 7% then the combined emitted light appears white and falls within the generally acceptable color boundaries of light suitable for illumination. Efficacy as high as 150 L/W has been reported for LEDs made in this area, but commercially available lamps generally have CRI Ra in the range of from 70 to 80.
White LED lamps made with this method typically have a CRI Ra of between 70 and 80. The primary omission from the spectrum being red color components and, to some extent, cyan.
Red AlInGaP LEDs have very high internal quantum efficiency, but due to the large refractive index mismatch between AlInGaP and suitable encapsulant materials, a lot of light is lost due to total internal reflection (TIR). Regardless, red and orange packaged LEDs are commercially available with efficacies higher than 60 L/W.
Additional information on LEDs for general illumination, shortcomings and potential solutions may be found in “Light Emitting Diodes (LEDs) for General Illumination” OIDA, edited by Tsao J. Y, Sandia National Laboratories, 2002.
U.S. Pat. No. 7,095,056 (Vitta '056) discloses a white light emitting device and method that generate light by combining light produced by a white light source (i.e., light which is perceived as white) with light produced by at least one supplemental light emitting diode (LED). In one aspect, Vitta '056 provides a device which comprises a light source which emits light which would be perceived as white, a first supplemental light emitting diode (LED) that produces cyan light, and a second supplemental LED that produces red light, wherein the light emitted from the device comprises a combination of the light produced by the white light source, the first supplemental LED, and the second supplemental LED. While the arrangement disclosed in Vitta '056 allows the CCT to be changed, the CRI and the usefulness of the device reduces significantly at lower color temperatures, making this arrangement generally undesirable for indoor general illumination.
The whiteness of the emission from a lighting device is somewhat subjective. In terms of illumination, it is generally defined as to its proximity to the planckian blackbody locus. Schubert, in his book Light-Emitting Diodes, second edition, on page 325 states, “the pleasantness and quality of white illumination decreases rapidly if the chromaticity point of the illumination source deviates from the planckian locus by a distance of greater than 0.01 in the x,y chromaticity system. This corresponds to the distance of about 4 MacAdam ellipses, a standard employed by the lighting industry. See Duggal A. R. “Organic electroluminescent devices for solid-state lighting” in Organic Electroluminescence edited by Z. H. Kafafi (Taylor and Francis Group, Boca Raton, Fla., 2005). Note the 0.01-rule-of-thumb is a necessary but not a sufficient condition for high quality illumination sources.” A lighting device which has color coordinates that are within 4 MacAdam step ellipses of the planckian locus and which has a CRI Ra>80 is generally acceptable as a white light for illumination purposes. A lighting device which has color coordinates within 7 MacAdam ellipses of the planckian locus and which has a CRI Ra>70 is used as the minimum standards for many other white lighting devices including CFL and SSL (solid state lighting) lighting devices. (see DOE—Energy Star Program requirements for SSL Luminaires, 2006).
The distance a color point is from the BBL can also be more easily defined in terms of Eu′v′. This is the length of a line between the color point of the lighting device and the nearest point on the BBL defined in the CIE76 color space. A Eu′v′ of 0.0012 is approximately equivalent to a single step MacAdam ellipse in the area near the BBL.
General illumination generally has a color temperature between 2,000 K and 10,000 K, with the majority of lighting devices for general illumination being between 2,700 K and 6,500 K. The white area is within Eu′v′ 0.01 (approximately 8 MacAdam ellipses) of the planckian locus and between 2,500 K and 10,000 K, and is shown in FIG. 1.